When a film is molded by melting a polyamide, the resultant film may contain inhomogeneous fine granular substances mixed therein. The granular substances are called fish eyes and have a melt viscosity (flow characteristic) obviously different from that of ordinary polyamide molecules. Occurrence of fish eyes is considered to be because of gels caused by denaturalization of polyamide such as particular increase of molecular weight or molecular growth of nonlinear chains (ternary polymerization) of polyamide molecules or of foreign substances mixed in polymer. When a polyamide containing gels or foreign substances is used as a molding material for transparent and relatively thin-wall molded articles of bottles, sheets, films or the like, not only the incidence ratio of defectives having a low commercial value in point of the outward appearance thereof owing to the presence of fish eyes increases, therefore resulting in productivity reduction, but also breakage of the molded articles obtained by molding a polyamide containing gels or foreign substances, which originates from the gels or the foreign substances, may be caused when the molded articles receive a shock.
In a polyamide containing a xylylene group such as a metaxylylene group, a paraxylylene group or the like (hereinafter referred to as “xylylene group-containing polyamide”), a radical may readily form at the benzylmethylene group, and therefore as compared with other polyamides such as nylon 6 or the like, the above polyamide has low thermal stability, and when heated in the presence of oxygen, it is readily oxidized to turn yellow. Accordingly, in the production process for a xylylene group-containing polyamide, an antioxidant is generally added to the raw material component for the purpose of preventing oxidative degradation and discoloration of the polyamide (for example, see PTL 1).
As an antioxidant for polyamide, a phosphate-type antioxidant such as a hypophosphite, phosphite or the like is generally used from the viewpoint of the yellowing preventing effect and the cost thereof. The phosphate-type antioxidant is handled as a powder having a mean particle size of 100 μm or so, and in polymerization to obtain polyamide, the antioxidant is, generally as a powder thereof, added to a molten dicarboxylic acid component.
However, it is known that as one reason for gel formation, the above-mentioned phosphate-type antioxidant is involved therein. The phosphate-type antioxidant such as hypophosphite, phosphite or the like absorbs oxygen in a reaction system, and hypophosphite is oxidized into phosphite, or phosphite is oxidized into phosphate. These may often act as a catalyst to accelerate amidation reaction, whereby not only polycondensation reaction is thereby difficult to control but also the above-mentioned particular increase of molecular weight or molecular growth of nonlinear chains of polyamide molecules is induced, therefore resulting in formation of gels.
As a means for preventing the phenomenon, it is known to add an alkali metal salt such as sodium hydroxide, sodium acetate or the like along with a phosphate-type antioxidant. However, excessive addition of an alkali metal salt inhibits amidation reaction to lower proceeding of polycondensation and, as the case may be, heat history in polyamide production may increase to form rather a large amount of gels. Accordingly, it is difficult to completely prevent gel formation by addition of an alkali metal salt.
On the other hand, as a means for removing gels and foreign substances from polyamide, there may be mentioned a technique of making a molten polyamide run through a filter in a molding process. However, gels may be pulverized into fine particles by a fluid pressure to pass through a filter, and it is difficult to completely remove gels. In turn, when the mesh diameter of a filter is reduced for removing finely-pulverized gels, the differential pressure through the filter increases so that the frequency of filter exchange increases and the continuous production time becomes short, therefore bringing about productivity reduction. Consequently, it is desired to prevent gel formation during polymerization rather than removing gels in a molding process.